AN ANTENNA STABILISATION SYSTEM

Abstract: An antenna stabilisation system includes a steerable mount for an antenna (10), actuators (12P, 12R, 12A) for steering the steerable mount. An amplitude detector (20) is connected, in use, in the signal path from the antenna (10) to detect the amplitude of the received signal. Control device (24) is connected to control the actuators (12P, 12R, 12A) and is responsive to the amplitude detector (20) to move the steerable mount towards the position in which the amplitude is maximised.

Description: "An Antenna Stabilisation System" This invention relates to an antenna stabilisation system, and especially a system for controlling and stabilising the aim of a directional antenna. The system is particularly but not exclusively useful for satellite communication terminals based on floating vessels, such as ships or floating platforms. Satellite communication systems generally achieve high transmission rates only by use of highly directional antenna dishes at the mobile station. It is therefore necessary to maintain the aim of the dish to within a few tenths of a degree, regardless of the motion of the mobile station. Where the mobile station is a ship, the dish must be stabilised to compensate for wave-induced motion of the ship. In the past this has generally been done by providing motion sensors, such as gyroscopic sensors, to detect motion of the ship and using this information to drive servomotors to maintain the aim of the dish. Such systems are complex, typically requiring sensors for each of roll, pitch, yaw, and X and Y movement, with attendant disadvantages of initial cost and a high maintenance requirement. Published United Kingdom patent application GB 2,173,347A describes a stabilising system which combines a motion sensor and servo system with the use of low-friction gimbals. This reduces the degree of intervention required from the control system and allows lower servo motor power to be used, but the above disadvantages of complexity remain. In addition, GB 2,173,347A and similar equipment, have the disadvantage that because the antenna relies on detection of motion of the ship on which it is mounted, no allowance is made for movement in the signal beam and this can not be compensated for using this type of system. In accordance with an aspect of the present invention, an antenna stabilisation system comprises a steerable mount for an antenna, actuator means for steering the steerable mount, amplitude detector means connected, in use, in the signal path from the antenna to detect the amplitude of the received signal, and control means connected to control the actuator means and responsive to the amplitude detector means to move the steerable mount towards a position in which the amplitude is maximised. Preferably, the steerable mount provides rotation independently about pitch, roll and azimuth axes, and the actuator means comprises servomotors arranged to produce rotation about these axes. Preferably, the system includes a feed system which comprises a deviation means to deviate the incoming beam to a discrete area of the antenna. Typically, the instantaneous values of signal level are proportional to the signal strength in a particular quadrant of the antenna. Typically, the deviation means comprises a cavity device having a number of cavities and a PIN diode for each cavity. Preferably, the cavity device has four cavities. In the preferred embodiment, the cavity device is a beam squint unit which operates on the same principles to that described in published European Patent Application No. 0,171,149A. Preferably, the system also comprises conversion means to convert the discrete values from the deviation means into a modulated single frequency signal, which may be amplitude modulated. The control means may suitably comprise a computer operating in real time, and may further include power amplifiers connected between the computer and the servomotors. The system may optionally include position sensors and/or rate sensors for one or both of pitch and roll, arranged to provide feed-forward signals to the respective servomotor. The foregoing components of the system are preferably mounted within a radome, the intermediate frequency signal being passed, for example by coaxial cable, beyond the radome for further processing. The system may include sensors within the radome monitored by the computer for sensing, for example, the presence of intruders or fire, or abnormal temperatures. From another aspect, the invention provides a method of controlling a steerable antenna, comprising monitoring the amplitude of the received signal, and adjusting the aim of the antenna in a direction to maximise the amplitude. Preferably, the method is carried out by sampling the amplitude of an intermediate frequency, the sampling suitably being repeated at intervals of about lOOμs. The intermediate frequency is preferably in the region of 70MHz. An example of an antenna stabilisation system in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 is a schematic block diagram of an antenna stabilisation system; and, Figs. 2A and 2B are an end view and a partial cross-sectional side view of a beam squint unit for use in the system shown in Fig. 1. A dish antenna 10 of conventional type is carried on a three-axis steerable mount (not shown) arranged roll over elevation over azimuth, the antenna 10 being attached on the roll axis. Each axis is driven by a servomotor 12 via a gearbox and toothed belt (not shown). The antenna feed system for the antenna 10 is substantially the same as the feed system described in published European Patent Application No. 0,171,149A. The feed system includes a beam squint unit 1 which is similar to that described in EP0,171,149A. The particular design of the beam squint unit 1 is shown in Figs. 2A and 2B. The beam squint unit 1 has four cavities 3 which are interconnected by a central conduit 4. Each cavity 3 includes a PIN diode 5 which can be switched on or off to switch the cavity on or off. The beam squint unit 1 is coupled to the dish antenna 10. The incoming signal at the antenna 10, typically at a frequency of 12 - 14 GHz (KU band frequencies), is deviated by the beam squint unit 1 to give discrete values of signal amplitude. Each of these discrete values is proportional to the signal strength in a particular quadrant of the dish antenna 10. The beam squint unit 1 outputs the discrete values to RF conversion equipment 2 which converts the discrete values to a 70MHz amplitude modulated signal. The 70MHz signal is passed by a coaxial cable 16 to below- deck equipment indicated generally at 18 and described further below. The 70MHz signal is also passed to a 70MHz receiver circuit 20 operating as an amplitude detector to produce an amplitude signal on line 22. Although the example described here is for an incoming signal frequency of 12GHz to 14GHz, the apparatus, and the invention in general, may be used with other bands with frequencies greater than about 1GHz, such as C band (4 - 6GHz), KA band (20 - 30GHz) or X band. The system is controlled by a computer 24, which may conveniently be any PC-compatible microcomputer. The computer 24 is connected via an interface circuit 26 to the line 22, and to power amplifiers 28 which supply the servomotors 12. The foregoing parts of the system are enclosed within a radome the boundary of which is indicated schematically at 30 but which is not physically illustrated in the drawing. The radome as such does not form part of the present invention and any suitable form of radome may be used. We prefer to use a 3.5m radome formed from a 'six piece dielectric sandwich material affording minimal losses at operational frequencies. Access to the enclosure is via a lockable rear hatch. Internal lighting and heating are provided, as are monitoring sensors for smoke, heat humidity and intrusion the sensors being connected to the computer 24 as indicated at 32. The computer 24 operates to sample the received signal amplitude at a high repetition rate, suitably every lOOμs, and actuates the servomotors 12 in such a manner as to maximise the amplitude. Suitable software for carrying out this optimisation function may readily be devised by those skilled in the art and is therefore not described herein. The response speed of the system may be improved by the inclusion of a feed-forward function using information derived from a pitch rate sensor 34P, a roll rate sensor 34R, and a dual-axis inclinometer for pitch and roll, 36. The signals from the roll rate sensor 34R permit the computer 24 to control the antenna 10 to maintain the integrity of the polarity of the antenna. The below-deck equipment in this embodiment includes a satellite communication modem 38 and multiplexing equipment 40 controlled by a rack-mounted PC-compatible computer 42. The computers 24 and 42 are interconnected by a direct RS232 or RS485 link 44 which allows the below-deck control station to be used for monitoring and controlling the equipment within the radome enclosure. Although described principally with reference to use in ship stations for geostationary communications satellites, the system of the present invention may be used in other steerable antenna applications, for example in fixed or mobile stations for receiving from low earth orbit satellites. Other modifications may be made within the scope of the invention.

Claims: CLAIMS l. An antenna stabilisation system comprising a steerable mount for an antenna, actuator means for steering the steerable mount, amplitude detector means connected, in use, in the signal path from the antenna to detect the amplitude of the received signal, and control means connected to control the actuator means and responsive to the amplitude detector means to move the steerable mount towards a position in which the amplitude is maximised. 2. A system according to claim 1, the system further comprising a feed system comprising a deviation means to deviate the incoming beam to a discrete area of the antenna. 3. A system according to claim 2, wherein the deviation means enables the feed system to sense signals indicative of the deviation of the antenna from bore site in the azimuth and elevation access to the control means. 4. A system according to claim 2 or claim 3, wherein the deviation means comprises a cavity device having a number of cavities. 5. A system according to claim 4, wherein a switching means is associated with each cavity. 6. A system according to claim 5, wherein the switching means comprises a PIN diode. 7. A system according to claims 2 to 6, wherein the cavity device has four cavities. 8. A system according to any of claims 2 to 7, wherein the system also includes conversion means to convert discrete values from the deviation means into a modulated single frequency signal. 9. A system according to any of the preceding claims, wherein the control means comprises a computer operating in real time. 10. A system according to any of the preceding claims, wherein the system also includes a motion sensor which senses roll and provides feed-forward signals to the control means to enable the control means to control the actuator means to steer the steerable mount in response to signals received from the motion sensor. 11. A control system according to any of the preceding claims, wherein the system also includes a pitch sensor to detect pitch of the system. 12. A system according to any of the preceding claims, wherein the system also includes an inclinator sensor to sense changes in the azimuth axis of the system.

Inventors: PIKE DAVID VERNON, PIKE, DAVID, VERNON
Applicants: EAST ANGLIAN ELECTRONICS LTD, PIKE DAVID VERNON, EAST ANGLIAN ELECTRONICS LIMITED, PIKE, DAVID, VERNON
Application year: 1994
Main classification: H01Q1
IPC codes: H01Q1